1. Field of the Invention
The present invention relates to a branching unit for a submarine telecommunication system.
2. Description of the Prior Art
Submarine systems were originally installed to connect two terrestrial optical and electrical power supply terminals (also known as power feed equipments (PFE)) on either side of the Atlantic Ocean, for example. A later improvement comprised three terrestrial optical and electrical power supply terminals connected by a branching unit. Accordingly, in terms of its electrical function, each terminal is connected to one end of an electrical conductor the other end of which is connected to one of the three terminals of the branching unit.
By extension, there can be more than three terrestrial terminals interconnected by the appropriate number of conductor segments and branching units, each of the latter always having three terminals.
In operation, and at a given time, a trunk current, typically of 1A, flows between two of the three terminals, the first connection fed with power in this way being defined as a trunk. A branch current flows between a marine ground attached to the branching unit and the last of the three terminals, which is isolated from the trunk, and the second connection fed with power in this way is defined as a branch.
There are therefore three configurations, depending on the terrestrial terminal chosen for the branch. The basic electrical function of a branching unit is to provide a choice between the three configurations.
A standard branching unit comprises three relays each comprising a contact and a coil that opens the associated contact when it is supplied with current.
The contacts are normally closed: the three terminals are connected and no current flows. If trunk current is sent from one of the trunk terminals, it flows in the coil that controls the contact associated with the “branch” terminal and connected to the last terminal. Opening the contact isolates the branch from the trunk. When the trunk current reaches a sufficient value, the third terminal is connected to the submarine ground.
At present, a reconfiguration, i.e. a change of trunk and branch, is initiated by the users of the terminals that send the relays of the branching unit currents suitable for the appropriate switching actions.
Human error can therefore create an unwanted configuration and likewise a transient disturbance can reduce the trunk current and create a fortuitous reconfiguration.
It has been proposed to add optically controlled additional relays to the branching unit, to lock or unlock the existing configuration, managing the isolation of the branch.
However, both the architecture and the use of the branching unit become more complex. New states are created between the passage from one configuration to another. The reconfiguration process is therefore difficult to use. Furthermore, the locking/unlocking functions are not offered for the contacts associated with the trunk.
Finally, because of the new relays, the branching unit does not protect the system against human error leading to closing of a relay when live, which is known as hot switching, and which is liable to damage the relay.
The object of the invention is to guard against accidental reconfiguration and hot switching and to simplify reconfiguration procedures.